Al-Sha'bi (may Allah have mercy on him) narrates from Masruq (may Allah have mercy on him) that the Prophet (peace and blessings be upon him) swore to Sayyidah Hafsah (may Allah be pleased with her) that he would not go near his slave-woman, and said: "She is forbidden to me." So the expiation for his oath was revealed, and he was commanded: "Do not forbid what Allah has made lawful." [Surah al-Tahrim: 1]
This is mursal, and we have narrated it with a connected chain in the previous chapter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15078